1. Field of the Invention
The present invention relates to an optical information recording medium for high density recording having a phase-change type recording layer, such as a rewritable DVD, and an optical recording method. Particularly, it relates to an optical information recording medium and an optical recording method, whereby the linear velocity dependency and the recording power dependency at the time of one-beam overwriting and the stability with time of recorded marks, are improved.
2. Discussion of the Background
With a compact disk (CD) or DVD, it is common that recording of binary signals and detection of tracking signals are carried out by utilizing a change in reflectance caused by interference of reflected lights from the mirror surface and the bottom of pits.
In recent years, phase-change type rewritable compact disks (CD-RW, CD-Rewritable) have been widely used as media interchangeable with CD. Further, also with respect to DVD, various proposals have been made for phase-change type rewritable DVD.
Such phase-change type rewritable CD and DVD utilize a change in the phase difference and the difference in reflectance caused by a difference in the refractive index as between an amorphous state and a crystalline state for detecting recording information signals. A usual phase-change medium has a structure comprising a substrate, and a lower protective layer, a phase-change recording layer, an upper protective layer and a reflective layer, formed on the substrate, whereby multiple interference of these layers can be utilized to control the difference in reflectance and the phase difference and to provide CD or DVD with interchangeability.
With CD-RW, within a range where the reflectance is lowered to a level of from 15 to 25%, interchangeability with a CD in the recording signals and the groove signals can be secured, and retrieving can be carried out by a CD drive having an amplifying system added in order to complement the low reflectance.
With a phase-change recording medium, erasing and rewriting can be carried out simply by modulating the intensity of a single laser beam. Accordingly, with a phase-change recording medium such as CD-RW or rewritable DVD, recording includes overwriting wherein recording and erasing are carried out at the same time.
For information recording utilizing a phase-change, it is possible to employ a crystal state, an amorphous state or a mixed state thereof, and a plurality of crystal phases may also be used. However, with rewritable phase-change recording media which are presently practically used, it is common that an unrecorded or erased state is a crystal state, and recording is carried out by forming amorphous marks. As a material for the recording layer, it is common to employ a chalcogen element, i.e. a chalcogenide alloy containing S, Se or Te.
For example, it may be a GeSbTe type containing, as the main component, a GeTe—Sb2Te3 pseudo binary alloy, an InSbTe type containing, as the main component, an InTe—Sb2Te3 pseudo binary alloy, a AgInSbTe type alloy containing, as the main component, eutectic crystal of Sb0.7Te0.3, or a GeSnTe type.
Among them, a system having excess Sb added to a GeTe—Sb2Te3 pseudo binary alloy, particularly, an intermetallic compound neighborhood composition such as Ge1Sb2Te4 or Ge2Sb2Te5, has been mainly practically used.
Such a composition is characterized by crystallization accompanying no phase separation which is specific to an intermetallic compound and has a high crystal-growth rate, whereby initialization is easy, and the recrystallization speed at the time of erasing is high. Therefore, as a recording layer showing practical overwriting characteristics, an attention has been drawn only to a pseudo binary alloy system or an intermetallic compound neighborhood composition (Jpn. J. Appl. Phys., vol. 69 (1991), p 2849 or SPIE, Vol. 2514 (1995), pp 294 –301).
On the other hand, with such a composition, metastable tetragonal system crystal grains will grow. Such crystal grains show distinct grain boundaries, and their sizes are not uniform and optical anisotropy is remarkable depending upon the orientation. Accordingly, there is a problem that optical white noises are likely to be generated.
Such crystal grains different in the particle sizes and optical properties, are likely to grow around amorphous marks, whereby there will be a problem that jitter of mark is likely to increase, or the optical characteristics are different from peripheral crystals, whereby they are likely to be detected as non-erased portions.
Accordingly, there has been a problem that it is impossible to obtain good retrieving characteristics in recording at a high linear velocity or in high density mark length modulation recording. Specifically, according to the specification for rewritable DVD, the shortest mark length is 0.6 μm, but if the shortest mark length is further decreased, jitter has been found to increase abruptly.
As a method for reducing jitter, there is so-called absorptivity correction. With a conventional four-layer structure, light energy absorbed by a recording layer is usually such that light energy Ac absorbed in a crystal state having a high reflectance is smaller than light energy Aa absorbed in an amorphous state having a low reflectance (Ac<Aa). Therefore, there is a problem that depending upon whether the original state was a crystal state or an amorphous state at the time of overwriting, the shapes, etc., of newly recorded marks are different, whereby jitter will increase.
Here, by adjusting the absorptivity of light energy in the crystal state and in the amorphous state to be substantially equal, the mark shape may be stabilized irrespective of the original state, whereby jitter can be reduced. Further, crystal requires an extra heat corresponding to the latent heat at the time of melting, and accordingly, it is preferred to adjust so that light energy is absorbed more in the crystal state (Ac>Aa).
To accomplish this relationship, a method may be mentioned wherein at least one layer of light absorptive nature is added to make a structure of at least 5 layers, so that part of light absorption in the amorphous state is absorbed by this absorptive layer. For example, an absorptive layer of e.g. Au or Si is inserted between the lower protective layer and the substrate or on the upper protective layer (Jpn. J. Appl. Phys., vol. 37 (1998), pp 3339–3342, Jpn. L. Appl. Phys., Vol. 37 (1998), pp 2516 –2520).
However, such a layer structure creates a problem in the heat-resisting property or the adhesion of the absorptive layer, whereby deterioration such as microscopic deformation or peeling tends to be remarkable by repetitive overwriting. Further, the storage stability is likely to be impaired, since peeling or the like is likely to occur.
Namely, it has been difficult with a GeTe—Sb2Te3 pseudo binary alloy recording layer to accomplish high densification while maintaining a conventional four-layer structure.
Further, with the GeTe—Sb2Te3 pseudo binary alloy recording layer, the birefringence has a wavelength dependency such that the shorter the wavelength, the smaller the real part and the larger the imaginary part. Accordingly, especially when a short wavelength laser beam is employed as a light source, it is difficult to accomplish the condition of Ac>Aa.
Therefore, in recent years, a AgInSbTe four component alloy has been employed as a material for the recording layer. The AgInSbTe four component alloy has a feature that an erasing ratio as high as 40 dB can be obtained, whereby a high linear velocity and high density mark length modulation recording can be carried out with the conventional four-layer structure without absorptivity correction.
However, high speed recording usually means that the crystallization speed is high and the recorded information is easily erased, whereby amorphous marks are likely to be crystallized and the stability of recorded marks with lapse of time is poor in many cases.
Further, a medium which is capable of recording and retrieving information at a high speed, has been demanded recently in order to shorten the time for recording or to speed up the transmission of information, as the quantity of information has increased. For example, a standard speed of CD (1-time velocity) is from 1.2 to 1.4 m/s. Whereas, CD-RW capable of recording at a speed as high as 4-times velocity has been commercialized, and CD-RW capable of recording at a speed as high as 8- to 10-times velocity is now desired.
On the other hand, as rewritable DVD, various types have been proposed or commercialized including DVD-RAM, DVD+RW and DVD-RW. However, rewritable DVD with 4.7 GB which is a capacity equivalent to read-only DVD, has not yet been practically developed.
Namely, a medium which is excellent in stability of marks and capable of recording short marks at a high speed is demanded.
However, it has been believed that the high speed recording and the stability of marks are incompatible properties and it is difficult to satisfy both of them at the same time.
The present inventors have extensively studied the principle of crystallization and conversion to amorphous state, and as a result, found an epoch-making medium satisfying all these properties at the same time.
Namely, it is an object of the present invention to provide an optical information recording medium which is capable of recording short marks very well at a high speed and excellent in the stability of marks, and an optical recording method.